2.7 Tidal diamonds

On charts, tidal information is given at specific points denoted by a purple diamond shape with a letter. A table, normally found at a corner of the chart, gives information on the tidal flow at each point.

The chart extracts below show Saint Mary’s Sound in the Scillies and the corresponding data table. The information is laid out in a fairly self-explanatory way:

Chart extract showing Saint Mary’s Sound. Note the location of diamond ‘C’

Tidal stream table from the same chart. Diamonds A, B and D are outside the area shown in the extract above.

Describe the tidal flow in Saint Mary’s Sound on July 4th 2017? We notice that the tidal stream tables refer to high water at Plymouth. We must begin by consulting a Plymouth tide table:

First few rows of a tide table for Plymouth.

On July 4th, we have the following times of high and low water, remembering to add the hour for British Summer time:

05:47 5.4 m High Water
12:01 1.2 m Low Water
18:14 5.6 m High Water

We note that the tidal range is 4.4 m, and the factor is 4.4/4.73 = 93%, which is close to springs. We will use the left hand column for diamond C which gives the rates at spring tides.

The table below shows data taken from tidal diamond C related to times on June 14. It has been created by entering the HW times for Plymouth on the ‘0’ row then counting backward and forward up and down the table:

Time relative to HW Bearing Time of day Rate Comment
-6 300 12:14 0.4 kt
-5 20 13:14 0.1 kt Tide is turning to the east
-4 109 14:14 0.7 kt
-3 110 15:14 1.2 kt
-2 111 16:14 1.5 kt
-1 111 17:14 1.6 kt
0 111 05:47 , 18:14 1.7 kt Tide runs at maximum speed east
+1 125 06:47 1.5 kt
+2 190 07:47 0.1 kt Tide is turning to the west
+3 275 08:47 1.3 kt
+4 272 09:47 1.7 kt Tide runs west at maximum rate
+5 265 10:47 1.0 kt
+6 292 11:47 0.7 kt

So, the tide starts flowing west a little after 07:47 in the morning. At around 09:47, it reaches a maximum speed in the westward direction of 1.7 knots. The speed drops until around 13:14, when the tide swings to the north, before starting to run east. It reaches the maximum speed eastward of 1.7 knots at around 18:14 in the evening.

Note the approximate times given – we only have data at each hour, so we can only estimate the times of slack water and maximum flow.